This invention relates to a stabilized absolute direct current laser inspection system, and more particularly to such a system with a single feedback loop which can compensate for changes in the laser light source and the light collection system such that the flaw signals produced by the system are influenced by the product inspected and not the system which is doing the inspecting.
In light inspection systems, radiation, for example, in the form of a laser beam, is scanned successively across the surface of material being examined and the intensity of the beam either reflected, transmitted or scattered from the material is detected by a receiver which includes a photomultiplier tube. The signal generated by the photomultiplier tube is called the product pedestal signal which varies in accordance with the characteristics of the material being examined thereby offering a method of determining whether flaws exist in the material or not by variations in the signal detected. The product pedestal signal also includes a certain dc level which will be present based on the intensity of the laser beam, system sensitivity, etc., regardless of the type of materials being examined or whether flaws exist in the material or not. This signal may be referred to as the background signal or the dc level of the system which in effect does not depend on product changes but rather on inspection system variations. For many applications, a relative change in the signal level is all that is required for determining flaws while in certain types of web products absolute changes are required for the purposes of identifying flaws. In the latter application, the background signal level of the system must be stabilized in order that changes in the product pedestal level will be reflected only by changes in the characteristics of the material being examined.